The Leslie sound systems are well known in the patent and commercial arts, and little need be said about them here. In general, either a horn is provided in front of a fixed loudspeaker, which horn rotates, or speakers themselves are rotated. This is acknowledged to produce generally a Doppler effect. In somewhat simplified analysis of a Leslie system, there will be heard the base frequency, and two additional frequencies, one being the nominal audio base frequency plus the frequency of the rotating system from the speaker approaching the listener, and a third frequency containing the audio base frequency minus the speed determined by the rotating system speaker moving away from the listener. This will provide three fixed frequencies.
There are disadvantages to the Leslie system introduced by the mechanical moving parts. A certain amount of noise always is to be expected from moving parts, there is a significant bulk which limits portability, the size of rotating parts is limited, and an organist cannot listen to such a system with earphones, nor can recordings be made thereof other than by way of a microphone. Leslie systems also always require separate, additional amplifiers, swell control potentiometers, and wood or plastic envelopes to prevent other sounds in the organ from being modulated by the rotating assembly. Strictly electronic efforts have been made to produce vibrato or celeste with varying degrees of success. One such system shown in the patent art in van der Kooij U.S. Pat. No. 3,883,752 and in Adachi U.S. Pat. No. 3,866,505 has achieved some degree of commercial success. In this system the audio tones from an organ are passed through three analog shift registers or bucket brigade delay devices. The three delay lines are respectively modulated by signals which are supposed to be 120.degree. out of phase with one another. However, the phase shifting is achieved by phase shifting networks, and as will be appreciated such networks can produce the exact degree of phase shifting required at only one given frequency. At all other frequencies the amount of phase shift will differ quite considerably, depending on frequency. Similar results have been achieved by the Wurlitzer Symphonic Presence Modulator incorporated in various contemporary Wurlitzer organs. A further such effort is made in Love U.S. Pat. No. 4,000,676, utilizing only two analog shift registers, and again relying on frequency shifting networks. Somewhat similar efforts are also made with use of either analog shift registers, or digital shift registers with analog-to-digital and digital-to-analog converters in Doughty U.S. Pat. No. 3,749,837. Older patent art of some relevance, but utilizing older artificial delay lines rather than shift registers includes Hanert U.S. Pat. No. 2,382,413 and Hanert U.S. Pat. No. 2,905,040.